Since an inorganic glass has excellent general properties such as excellent transparency and low optical anisotropy, the inorganic glass has been widely used in many fields as a transparent material. However, the inorganic glass has drawbacks such that it is heavy and easily breaks, and has bad productivity when producing a product by molding and processing. As a result, a transparent organic polymer material (optical resin) has been used as a material in place of the inorganic glass. As the optical part obtained from such an optical resin, there are, for example, a plastic lens such as a spectacle lens for vision correction or a camera lens of a digital camera, and the like. The optical parts have been put to practical use and have come into use. In particular, for the purpose of use in a spectacle lens for vision correction, the plastic lens is light-weight and hardly broken, and can be tinted for granting great fashionability, as compared to the lens made of an inorganic glass. Making good use of such merits, the plastic lens has been widely used.
In the past, a crosslinking type resin obtained by casting polymerization of diethylene glycol bis(allyl carbonate) hereinafter, referred to as a DAC resin) as an optical resin used for a spectacle lens under heating has been put to practical use. It has merits such that transparency and heat resistance are excellent, and the chromatic aberration is low. Due to such merits, it has been used the most for a general-purpose plastic spectacle lens for vision correction. However, there are problems like poor wearing comfort and fashionability because the central or edge thickness of the plastic lens becomes large due to the low refractive index (nd=1.50). Therefore, a resin for a plastic lens with a high refractive index capable of solving these problems has been demanded and developed accordingly.
During such a trend, since polythiourethane containing a sulfur atom obtained by casting polymerization of a diisocyanate compound with a polythiol compound is excellent in its transparency and impact resistance, while achieving highly superior characteristics such as attaining a high refractive index (nd=1.6 to 1.7) and having relatively low chromatic aberration, and the like, polythiourethane has been used for the purpose of a high-quality plastic spectacle lens for vision correction in which the thickness is thin and its weight is light.
On the other hand, in a trend to pursue an optical resin having a much higher refractive index, there have been proposed several resins such as a transparent resin obtained by polymerization of a compound having an episulfide group (Patent Documents 1 and 2) or a resin obtained by polymerization of a compound having a thietane group (Patent Document 3), or a resin obtained by polymerization of a Se-containing compound (Patent Document 4). However, the transparent resin obtained by polymerization of a compound having an episulfide group has a problem in mechanical properties; the compound having a thietane group has a problem in polymerizability; and the resin obtained by polymerization of a Se-containing metal compound has a problem in safety. Therefore, they have been demanded for further improvement. In recent years, there has been demanded and developed an optical resin with a high refractive index, having required general properties (transparency, thermal properties, mechanical properties, and the like) as a plastic lens, while attaining a much higher refractive index (nd) exceeding 1.7, for example. Under these circumstances, there has been newly developed a metal-containing thietane compound, and there has been proposed an optical resin having a high refractive index (nd) exceeding 1.7 (Patent Document 5).    [Patent Document 1] Japanese Patent Laid-Open No. 9-110979    [Patent Document 2] Japanese Patent Laid-Open No. 11-322930    [Patent Document 3] Japanese Patent Laid-Open No. 2003-327583    [Patent Document 4] Japanese Patent Laid-Open No. 11-140046    [Patent Document 5] Pamphlet of International Patent Publication WO 2005-095490